Let us make an in-depth study of water deficit and drought resistance in plants. After reading this article you will learn about A. Water Deficit and Drought Resistance in Xerophytes and B. Water Deficit and Drought Resistance in Mesophytes.
One of the most common environmental stresses encountered by plants during certain periods of the year is water deficit or negative water potentials. In agricultural conditions, prolonged drought periods and scanty or erratic rainfall cause widespread damage to crop plants. According to Kramer, P.J. (1969), drought resistance may be defined as “referring to various means by which plants survive periods of environmental water stress”.
On the basis of their response to available water, the plants are usually classified into three categories:
1. Hydrophytes i.e., the plants growing at places where water is always available e.g., in a pond.
2. Xerophytes i.e., the plants growing at places such as deserts, where water is scarce at most of the time.
3. Mesophytes i.e., the plants growing at places where water availability is intermediate.
Obviously, among these categories of plants, the xerophytes and mesophytes are prone to water stress and they adopt various means to cope with environmental water stress. Desert xerophytes have usually been studied by physiological ecologists whereas extensive research work has been done on water stress in mesophytes by foresters and agriculturists.
A. Water Deficit and Drought Resistance in Xerophytes:
Xerophytes are basically drought resistant either because:
(i) Their protoplasm can just tolerate or endure extreme and prolonged desiccation (dehydration) without being killed or,
(ii) They possess structural (morphological) or physiological characteristics to avoid or postpone lethal level of desiccation.
(a) Desiccation Tolerance:
Among xerophytes, some plants such as many mosses, lichens, few ferns and some seed plants have protoplasm which can tolerate or endure extremely negative water potentials (or extreme water stress) without being killed. Such plants are said to exhibit desiccation tolerance or hardiness.
Among seed plants, examples are various desert grasses and shrubs such as creosote bush (Larrea tridentata) and sagebrush (Artemesia sp.). In creosote bush, the water content may drop upto 30% of the final fresh weight before the leaves die (whereas in most plants water contents below 50-75% are lethal). Protoplasmic properties of desiccation tolerant plants are however, not clearly understood.
The degree of desiccation tolerance may differ among various species of grasses, shrubs and trees of less arid habitats. Some plants such as Selaginella lepidophylla (resurrection plant), some grasses and ferns e.g., Polypodium have remarkable ability to dry out and recover and becoming fresh immediately upon watering.
Although some desiccation tolerant species may also possess characters of drought avoiders such as small leaves, sunken stomata etc., but the main mechanism of drought resistance lies in their protoplasm which just endures it.
(b) Desiccation Avoidance or Postponement:
This can be achieved by plants in various ways:
(i) Drought Escapers:
Many desert annuals escape drought by completing their life cycle before severe water stress develops. Their seeds remain dormant during dry season. These seeds germinate, grow and flower within a few weeks after the rains have wetted the soil to considerable depth.
(ii) Water Spenders:
Some plants such as palms growing at an oasis, develop roots that go deep down to the water table or mesquite tree (Prosopis glandulosa) and alfalfa (Medicago sativa) that have roots which extend up to 7 to 10 metres down to the water table, aggressively consume water and avoid drought. Such plants are known as water spenders and in-fact they never face extremely negative water potentials (or water deficits). A combination of potentially deep rooting species and soil conditions favourable for deep rooting is advantageous for water spenders to avoid drought.
(iii) Water Collectors:
Under moderate water stress, some succulent plants such as cacti, century plant (Agave americana) and other CAM plants resist drought by storing water in their succulent tissues. Such plants which use water conservatively are known as water collectors. Because of thick cuticle and stomatal closure during the day, the loss of water is lesser in such plants so that they can survive longer dry periods.
Some succulent plants which are subjected to periodic drought are known to exhibit facultative CAM. Under water stress conditions, they show CAM, but they switch over to C-3 mode of photosynthesis when enough water is available.
(iv) Water Savers:
Some non-succulent desert plants show many adaptations to reduce water loss through transpiration such as smaller laminar area, sunken stomata, thick hairy covering on surfaces of leaves and shedding of leaves during dry periods. Such plants are called as water savers.
(v) Osmotic Adjustment:
As a result of increasing water stress in many xerophytes including those of above mentioned categories, certain organic compounds such as amino acid pro-line and sugar alcohol sorbitol etc., accumulate in the cytoplasm of cells. These substances (compatible solutes) lower the osmotic potential and also the water potential of cells without damaging enzyme functions. This drop in osmotic potential which helps in maintaining plant water balance has been called as osmotic adjustment or osmoregulation by Morgan (1984).
B. Water Deficit and Drought Resistance in Mesophytes:
In agricultural conditions, prolonged drought periods and scanty rainfall cause widespread damage to crop plants all over the world. To cope with this problem, regular irrigation becomes necessary that requires huge amount of money and which may be influenced in turn by researches in stress physiology.
Therefore, extensive research work has been done on water stress in mesophytes by agriculturists and foresters whose main concern is in the extent to which water economy might influence crop yield rather than severe water stresses endured by xerophytes.
Theodore C. Hsiao (1973) has outlined the sequence of events which occurs during gradual water stress in plant growing in substantial volume of soil as discussed below. It is noteworthy that later events may be indirect responses to earlier events rather than direct response to water stress itself.
1. Decreased Cellular Growth and Leaf Area:
One of the earliest responses of plants to water stress is lowered cell turgor that results in decreased cellular growth or cell enlargement in tissues that normally grow rapidly. By decreasing the external water potential (Ψ) by -0.1 MPa or even less, a perceptible decrease in cellular growth or cell expansion and thus root and shoot growth can be observed. As a result, there is remarkable decrease in leaf area also that transpires less water and is supposed to be first line of defense against drought (Fig. 23.1).
Decrease in cell expansion is usually followed by reduction in cell wall synthesis. Protein synthesis may also be equally sensitive to water stress and appears to be controlled at translation level. Water stressed plants tend to be rehydrated at night. Consequently, substantial leaf growth occurs at night in such plants.
2. Inhibition of Protochlorophyll Formation and Enzymes Activities:
At slightly more negative water potentials (or increasing water stress), protochlorophyll formation is known to be inhibited in many plants. The activities of certain enzymes such as nitrate reductase, phenylalanine ammonia lyase and others decrease sharply. Nitrogen fixation and reduction also decrease. Cell division is also inhibited with water stress.
3. Accumulation of ABA and Stomatal Closure:
In water stressed plants, level of phytohormone abscisic acid (ABA) increases manifold in leaf tissues. This leads to closure of stomata and reduced transpiration, thus providing resistance to water stress. To some extent, ABA is also known to accumulate in roots where it increases conduction of water, thereby reducing water stress in shoots.
Externally applied ABA to leaves of normal plants is also known to induce stomatal closure and studies done with drought sensitive and drought resistant cultivars of crop plants have confirmed the role of ABA in resistance of mesophytes to water stress or drought.
4. Stimulation of Leaf Abscission:
In water stressed plants, enhanced synthesis of endogenous phytohormone ethylene is known to stimulate abscission of leaves.
5. Accumulation of Compatible Solutes:
At relatively mild water stresses (Ψ = -0.2 to -0.8 MPa), the amino acid proline begins to accumulate rapidly in cytoplasm. Sometimes, its concentration may become as high as 1% of dry weight of tissue. Other amino acids such as glycine betaine and sugar alcohol sorbitol also accumulate in cytoplasm.
These organic compounds do not interfere with enzyme functions and decrease the water potential of the cells without accompanying decrease in their turgor. These organic compounds are called as compatible solutes (or compatible osmotica) which contribute to osmotic adjustment of cells and help in maintaining plant water balance and thus increase resistance of plant to water stress.
6. Decreased Photosynthesis, Translocation of Assimilates and Respiration:
When levels of water stresses are higher (Ψ = -1.0 to -2.0 MPa), photosynthesis, translocation of assimilates (organic solutes) in phloem and respiration are markedly inhibited. In some cases, there may be in-fact an increase in respiration rate but at still higher level of water stress (Ψ = -5.0 MPa), respiration also drops sharply and the plant wilts.
Water stressed plant can be recovered on watering if the stress was not severe and the plant has not been permanently wilted. But the growth and yield of water stressed plant is always lower in comparison to unstressed one. Therefore, responses of plants to water stress may appear elastic at first glance, but in-fact in terms of final yield they are plastic.